Packaged circuit boards for electronic instruments are typically coated by a moisture-proof, liquid insulating film to protect the circuit boards from moisture, electric leakage and dust. Preferably, the moisture-proof insulator films are what are known as conformal coatings, such as acrylic, polyurethane or epoxy synthetic resins dissolved in a volatile solvent. When applied to a clean printed circuit board, a uniform thickness insulative resin film is formed as the solvent evaporates on a continuous basis.
One method of applying coatings of moisture-proof insulators to printed circuit boards in mass production processes has been the spray method, in which the moisture-proof insulator is applied to the printed circuit board by spraying techniques such as disclosed in U.S. Pat. Nos. 4,753,819 and 4,880,663, both to Shimada, which are owned by the assignee of this invention. Various film coaters or dispensing apparatus have been employed for this purpose, which typically include a plunger to open and close a discharge outlet through which the coating material is emitted and then applied onto the circuit board or other substrate. The plunger in such dispensers is usually moved to an open position with pressurized air acting on a piston carried by the plunger, and returned to the closed position by a return spring. A flow control mechanism, such as a computer controlled solenoid valve, is used to control the flow of pressurized air from a source to the dispenser. Conventionally, the valve is separated from the dispenser by as much as 6 feet or more of air line.
When coating printed circuit boards, it is often desirable to only coat selected areas of the board. One method involves moving the dispenser to the selected area, opening the discharge outlet when it is over the selected area, effecting relative movement between the discharge outlet of the dispenser and the circuit board in order to coat the selected area, closing the discharge outlet, and repeating this process for each area to be coated. Various problems have been encountered in previous attempts to selectively coat in this manner.
One problem has been the response time of the dispenser, i.e., the time it takes for coating material to begin or stop coating the target substrate after a computer or other controller has signaled the valve mechanism associated with the source of pressurized air to open the plunger of the dispenser or allow it to close. Previous dispensers had response times that were only fast enough to coat relatively large selected areas without having to mask the parts of the circuit board to be left uncoated. Attempts to coat smaller areas of printed circuit boards have generally required additional programming of the computer control, slowed down production, and/or resulted in less control of the sharpness of the boundaries of the coating pattern produced and the repeatability of the pattern from board to board.
Other problems plaguing such dispensers, independent of the slow response time, involve distortions of the coating pattern which are often referred to as "hammering", "tapering" and "drooling". Hammering is typically evidenced by a broadening of the coating pattern beyond the desired pattern area when the discharge outlet of the dispenser is opened and/or closed. Tapering is displayed as a narrowing of the coating pattern when the discharge outlet is opened. Drooling is evidenced by a tail of the coating material being deposited on the substrate outside the pattern area when the discharge outlet is closed.